1. Field of the Invention
This invention relates to method and apparatus for periodically adjusting the position of an endless web.
2. Description of the Prior Art
Endless webs trained about rollers have a tendency to become misaligned. In an extreme case, they run off the ends of such rollers. A misaligned web can be damaged by abrasion with roller side guides. The lateral wandering of a web at a roller is caused by crosstrack frictional forces. Prior web apparatus for aligning a flexible moving web about an endless path include two types of cylindrical rollers, or supports: (1) laterally constraining rollers and (2) laterally nonconstraining rollers. Laterally constraining rollers often include web-engaging portions which engage and center the moving web. These portions are spaced a distance apart which is slightly larger than the width of a moving web. Laterally nonconstraining rollers change their spatial orientation in response to the lateral movement of the web. There are active and passive systems which use such nonconstraining rollers. In the active category, servo-controlled rollers laterally position a web under the control of an external mechanism which senses web misalignment and triggers a compensating mechanism to return the web to an aligned position. See for example, U.S. Pat. No. 3,973,446 to Vasilantone. Passive systems often include castered and gimballed rollers having edge guides.